The prior art contains many pumping unit designs, all of which have at least some shortcomings, and only a few of which have isolated features or operational characteristics which are similar to any of those of the subject invention.
Most obvious among these shortcomings is the absence of an efficient stroke reversal mechanism in combination with a linear drive mechanism which provides a relatively high stroke velocity, eliminates power peaks, and allows a smaller, more efficient motor for either long or short stroke units.
Within the field of the present invention, only applicants' U.S. Pat. No. 5,536,150 incorporates features which furnish a stroke reversal capability and eliminate parasitic rod string oscillation due to stroke reversal. All of the other prior designs cause the reversal of the polish rod and upper end of the rod string with little regard for the current forces upon the lower and central portions of the rod string due to inertia and rod stretch.
This often results in the upper end of the rod string reversing from upward to downward movement while the lower end of the rod string is still moving upward, and, respectively, reversing from downward movement to upward movement at its upper end while its lower end is still moving downward.
The beam unit which utilizes a flywheel type counterbalance has been by for the most popular and successful pumping unit for many years. Its rotary drive connection to the beam produces a smooth reversal, and its flywheel transfers kinetic energy from one stroke to the next. Its drive motor is engaged constantly, which is an advantage, although there are power peaks, and a large motor is required.
The beam unit does have serious drawbacks. The required gearbox is heavy and expensive, and the stroke length is limited to about twenty feet. The polish rod and top end of the rod string reverses direction without regard for existing conditions of stretch or contraction in the rod string, and parasitic rod string oscillation is often a problem. Stroke velocity is then limited and rod string problems are a factor.
The beam unit which utilizes counterweights attached to the beam has had some success in the smaller capacity units. With this unit there is a very limited transfer of energy from each upstroke and downstroke to the succeeding stroke, and as the stroke velocity increases, the efficiency of the unit decreases, which is perhaps the reason for its limited success.
Linearly driven pumping units, both mechanical and hydraulic, are very well represented in the prior art.
Applicants' U.S. Pat. No. 5,536,150 describes a hydraulic/pneumatic stroke reversal system which requires the cyclic opening and closing of at least one hydraulic valve.
This design has many of the same desirable operational characteristics that the subject invention does: energy is transferred to the following stroke, reversals are smooth and at the instant of greatest rod stretch or contraction, and a higher stroke velocity and more efficiency are provided.
There are many designs of linearly driven mechanical pumping units which use a mechanical counterweight. Very few of these designs show us a reversal system which transfers energy, and the problems of reversals are doubled because of the inertia of the counterweight mass. Almost all of these designs are for long stroke units, in which the reversal problems and power losses are minimized by a low stroke velocity and low cadence.
One such unit which has enjoyed commercial success is the RotaFlex.RTM. unit which is marketed by Energy Ventures, Inc. It uses an endless chain drive, and the reversal characteristics are determined by the size of the drive sprocket. At least some energy is transferred from one stroke to the next, but the reversals do not make allowance for rod stretch or contraction. This unit is referred to as a long, slow stroke unit.
At least one prior mechanical design provided an efficient and effective transfer of kinetic energy from one stroke to the next and also caused reversal of the pumping unit at the instant of maximum rod stretch and of maximum rod counteraction, respectively.
The design used large spiral drive and counterweight pulleys, which were awkward and expensive, and which caused accelerated wear on the drive cables because of lateral misalignment of the pulley grooves. This design enjoyed a limited commercial success.
There are many hydraulic drive pumping unit designs in the prior art, all of which, except for applicants' U.S. Pat. No. 5,536,160, have no stroke reversal system and do not effectively transfer energy or reverse the pumping unit stroke at the instant of maximum rod stretch and maximum rod contraction, respectively.
Most of these hydraulic designs utilize a pneumatic counterbalance, and none of them shows us a method for preventing a variation in the force supplied by the counterbalance from bottom to top of the stroke, which results in a variation during the stroke, in the force required from the drive system.
Many of these pneumatic counterbalance systems select a value for this counterbalance force, and a size for the counterbalance pressure vessel, or "compression space," which only partially offsets the discrepancy between the drive power required for the upstroke and for the downstroke, respectively.
Many prior hydraulic designs require extensive control systems, and a large number of components, and all of them, on average, use a counterbalance pressure storage member that is several times the size of the hydraulic drive cylinder.
None of the prior designs which utilize a pneumatic counterbalance have demonstrated a method for reducing the size of the required compression space, or pressure vessel volume which is in addition to the piston swept space of the counterbalance cylinder or accumulator, to a minimum. This compression space volume for prior designs has varied from ten times, to a minimum of three times, the volume of the piston swept portion of the counterbalance system.
Few if any of the prior art hydraulicly driven units are suggested for permanent installation of short stroke units, no doubt because the many reversals, without benefit of energy transfer from one stroke to the next, require a low cadence, along with limited production and a lack of efficiency.
Stroke velocity is limited in prior short stroke units because power must cease far enough before the end of the upstroke and downstroke that the production drag, a force approximately equal to one-half the fluid column weight, will stop movement.
The motor must be large enough to begin and accelerate each upstroke and downstroke without assistance, and also to produce proper production power before it is required to shut down some distance before the end of each stroke.
As stroke velocity and cadence of these units increases, the motor must progressively furnish more total power for acceleration and production, in a shorter time because of the increased cadence, and in a smaller percentage of each stroke because of the increased portion of each stroke required to then stop movement of the system by means of production drag.
The motor for these prior short stroke units, then, is made progressively larger as the stroke velocity and/or cadence increases, with an accompanying progressive decrease in efficiency, for units of a particular size.
Because of the above considerations, most efforts in the field of linearly driven pumping units have been directed toward long, slow stroke units, in which the problems associated with stroke reversal are minimized.
The normal variation during the stroke of the force furnished by a pneumatic counterbalance tends to assist in reversals, but the results are erratic and ineffective.
The prior long stroke units, however, are also inefficient when compared to a unit according to the subject invention which is one half their size and stroke length, which possesses a stroke reversal capability and operates at the same stroke velocity and double the cadence, and achieves the same production.
Of the prior designs within, or close to, the field of the present invention, only applicants' U.S. Pat. No. 5,536,150 offers a stroke reversal capability and a reversal which occurs at the instant of maximum rod stretch or contraction. The hydraulic/pneumatic reversal system of U.S. Pat. No. 5,536,160 requires hydraulic valving for cyclic control, and its reversal forces are controlled and transferred by means of hydraulic circuitry.